Wavefront errors can be described and represented mathematically, for instance, by the so-called Zernike polynomials. They are detectable, for example, by wavefront aberrometers. In this case, the wavefront errors may be caused by individual imaging errors or by superposition of different imaging errors of single or multiple optical elements of an optical system. In many cases, individual optical elements have a dominant influence on the imaging errors of the system, and the wavefront deformation caused thereby has a typical appearance, which can be countered by active deformation of the surface of one of the optical elements or of a mirror additionally arranged in the system for this purpose.
In order to counter imaging errors, a large number of solutions are known from the prior art which use active surface deformation of a planar optical element, usually a mirror. They differ substantially in the location where forces are applied into the planar optical element. In some of these solutions, forces are applied to the peripheral surface of the planar optical element or in the edge region adjacent to the peripheral surface. In others, forces are applied along or near the central axis or the axis of symmetry, respectively, of the planar optical element.
Using a correction device in accordance with US 2012/0275041 A1, an imaging error of known evolution can be corrected within an optical instrument. For this purpose, a deformable mirror is introduced into the optical path of the arrangement, along which beams of electromagnetic radiation propagate. Forces are applied to the peripheral edge of the deformable mirror and introduced into the mirror such that the latter is deformed as a function of its contour as well as the location of force introduction and the vectors (amount, direction) of the forces introduced. The deformation of the mirror caused thereby and the resulting local change in the reflection behavior, as a consequence of optical path differences, enable correction of any wavefront errors occurring.
Specifically, the aforementioned US 2012/0275041 A1 proposes to select the contour of the mirror as a function of the imaging error to be corrected, e.g. round so as to correct the focus position, or elliptical so as to correct the focus position and astigmatism. Despite the same forces acting on the perimeter, this allows locally differing flexing moments to be applied to the surface, depending on the distance of the force application from the center of the mirror. It is proposed therein that, as a means of force application, an intermediate plate having the same contour as the mirror be connected to the peripheral edge of the mirror by a ring and that a force be introduced centrally into the intermediate plate, e.g. by means of a piezo actuator, said force then acting on the periphery of the mirror. In order to differentiate the force effect along the periphery of the mirror, it is proposed to design the intermediate plate differentially in thickness or/and to introduce the forces eccentrically into the intermediate plate. This solution allows different flexing moments to be generated in the mirror along the periphery. The ratio of the flexing moments with respect to each other is predetermined by the design of the contour of the mirror and is thus no longer variable.
U.S. Pat. No. 7,229,178 B1 discloses a deformable mirror whose circular or oval mirror plate can be bent parabolically via an internal (smaller) and an external (larger) annular support. For this purpose, the mirror is placed between the annular support. The required force is introduced by a mechanical actuating element. The actuating element acts directly or indirectly, via a lever, on the annular supports. Force introduction via the level and parallel annular supports is always effected centrally with respect to the axis of symmetry of the mirror. This merely allows the parameters of a paraboloid surface to be influenced. Again, no locally differentiated application of force is possible.
In a device known from DE 601 16 322 T2, aberrations are also compensated for by application of force onto a mirror. For the application of force, there are provided at least one active actuating element and two so-called force transmission rods. The force transmission rods each have their ends connected to the mirror via a respective deformation element, thereby applying the same force into the mirror at different points. A differentiated application of force at different points is not possible.
DE 196 28 672 C2 discloses a mirror comprising a deformable mirror plate held by its edge as well as comprising an actuating mechanism acting on the rear surface of the mirror plate, said actuating mechanism comprising a plurality of springs connected in parallel, which act on the rear surface of the mirror plate, and comprising an adjusting means for adjustment of the spring force. In this case, a force is applied into the center of the mirror and into eccentric points of action which are arranged in radially opposite pairs. The introduction of force can be adjusted only jointly for all points of action.
DE 42 36 355 C2 discloses an adaptive membrane mirror, comprising a round membrane clamped concentrically between two annular blades and comprising an actuator which is provided for axial displacement of the two annular blades relative to each other. A force introduced through the actuator is introduced on the rear surface of the membrane by one of the annular blades, while the annular blade contacting the membrane acts as a counter bearing. The adjustment travel of the actuator is converted directly into a bending travel by which the membrane between the two annular blades deviates from a planar surface. The device is limited to curving a mirror in rotation-symmetric manner. This results in a spherically curved mirror with a radius of curvature of greater than approximately one meter.
A likewise eccentric introduction of force into a mirror plate is known from EP 1 118 987 B1. Here, a translationally acting actuator acts on the mirror plate at eccentric positions behind the mirror surface, which positions are diametrically opposite one another with respect to the center of deformation. Due to the mirror plate being axially mounted, this eccentric force introduction—as opposed to the central force introduction—results in a leverage effect which, at the same axial stroke, leads to a stronger curvature with respect to the edge of the plate than the same stroke applied to the center of the mirror plate. This results in a less parabolic but rather more arc-shaped curvature of the mirror surface. The eccentric points of action of the actuator may be discretely staggered or arranged continuously along a track. The track need not be circular, but may also be oval, resulting in different radii of curvature.
DE 10 2014 208 984 A1 discloses an assembly comprising a support and an element which is adjustable and fixable with six degrees of freedom in said support, in particular a facet mirror for use in a micro-lithographic projection exposure system. The six degrees of freedom are ensured by three balls which are each supported between a concave ball segment surface formed on the support and a concave ball segment surface formed on the element. A potentially impinging beam can only be influenced here in terms of its deflection.
EP 0 710 551 B1 describes a device for producing a printing stencil, said device comprising focusing optics with at least one elastically deformable mirror diaphragm. Actuating means are provided which set a curvature of the mirror diaphragm as a function of an actuating signal. As for the actuating means, it is disclosed that they are preferably piezoelectric or magnetostrictive actuating means, which preferably act only on the central rear part of the mirror diaphragm or on an annular blade.